1. Field of the Invention
This invention relates in general to supporting a casing in a wellhead, and in particular to a system for supporting the casing under an emergency basis when the casing is stuck.
2. Description of the Prior Art
Wellheads of the type concerned herein have a wellhead housing at the top of the well. After drilling the well to the desired depth, a string of casing is lowered into the well. A casing hanger secured to the upper end of the casing lands on a landing shoulder in the wellhead. Cement is pumped down the casing to flow up the annulus around the casing to cement it in place. The well may be subsequently drilled deeper or completed with tubing.
Occasionally, an emergency condition will exist. This occurs when the casing becomes stuck in that it cannot move downward or upward. A standard casing hanger secures by threads to the upper end of the casing, and cannot be employed in its normal manner because it would be above the landing shoulder in the wellhead housing.
There are various methods and devices used in this situation. Typically they involve cementing the casing, then cutting the casing off in the wellhead housing. Then the operator inserts slips over the casing and sets them on the landing shoulder in the wellhead housing. The operator tensions the casing. Then, a seal or packoff is placed between the wellhead housing and the exterior of the casing.
One disadvantage of the prior art method is that it may result in the blowout preventer being inoperative before the casing hanger seal is installed. The blowout preventer mounts above the wellhead housing and seals to the exterior of the casing. Normally the operator must cut off the casing within the wellhead, requiring removal of the blowout preventer. A danger exists that a blowout could occur.
It has not been possible to place the slips over the casing prior to cutting the casing because of the existence of a casing collar above the landing shoulder in the wellhead housing. The casing collar is of a larger diameter than the inner diameter of the slip assemblies utilized previously.
Also, when sealing, the seals need to seal on the rough exterior of the casing. If metal-to-metal seals are employed, this is very difficult. Metal seals seal best against very smooth surfaces.